Already have a 6 wt outfit and convinced myself I also "need" a 4 wt outfit to enhance my enjoyment when fishing for panfish, smaller (typically < 3 lb) largemouth bass and trout. Just purchased a St. Croix Imperial 4 wt fly rod and would appreciate your guidance on reel selection.

My impressions are:
a) Since the St. Croix Imperial is a light weight rod, it's important that the reel also be light weight for optimum balance.
b) In setting up a lightweight fly fishing outfit, money is better spent on the rod and line. Reel is basically a line holder at this line weight (#4).

That said:
1) Is it fair to say that the most important feature is low weight? If so, is it reasonable to think the reel should weight four ounces or less?
2) Should I also consider reels with click and pawl drag (assuming it results in a lighter reel) or am I better off with a disc drag to better handle larger than expected fish?
3) Is it reasonable to assume I can spend less than $100 to for a suitable reel?

Just my $.02 but be careful with click/pawl. While you can land quite sizable fish on a click/pawl just be careful that it isn't sloppy. If you get a sloppy click pawl you will get backlash when you strip line.

I believe you can get some reels for $100 or so like a Lamson Konic and they are a fine reel with a good drag.

I was at the Bears Den mini-show yesterday and they had some of these in the Orvis booth. They will be rolling them out in the US later this spring or summer. Very nice click and pawl for not a lot of money. The spring tension has a couple of different adjustment settings, depending on your preference for a gentle or stronger check.

get the ats I. you don't really need much backing on a 4 weight and the smaller size will suit that rod better.
on a side note, you may want to consider the ats fly line as well. i'ts great line at a good price